WHAT: Defining a population can be tricky


  • A group of organisms from one species
    • abundance
    • sex ratios
    • age structure


  • Defining the population boundary…
    • a lake
    • ecosystem
    • country or state


  • Aphid population: leaf, plant, grassland, ecosystem?
    • depends on the study and the questions

WHY: Understanding population structure (Demography)



  • # of individuals is how we measure populations


  • Age of a population, peak reproductive ages


  • Sex ratios determining mating potential


  • Measuring, predicting and evaluating population sizes guides ecologist
    • is it growing, declining, holding steady?

Case Study: Deer population size


Case Study: Deer population density


Case Study: Zombie and population density


WHO: Definition of an individual can be tricky



  • Unitary organism: forming a single entity
    • deer, bird, spider, etc.


  • Modular organisms are harder
    • bacteria, algae, corals
    • plants: ferns grow by rhizomes


  • Some modular organisms may themselves be populations

HOW: Estimates from representative samples






  • Lessons from Lab:
    • Census techniques
    • Mark & Recapture
    • Spatial component
    • Temporal component

Life History: More than just birth and death

Reproductive effort and life cycles: Long lived organisms

Semelparous: One-off reproduction


Populations and life cycles: Annuals & Ephemerals

Life History: Sexual conflict



  • Annuals vs Perennials
    • not just plants









  • When and how to spend your resources
    • reproduction comes at a cost of growth
    • flowers + seeds = ↓ leaves and roots
    • are enough resources available to support pregnancy?
    • surviving vs growing the population…

Populations and life cycles: Dormancy


  • Annual organism may spend part of the year dormant
    • Seeds, Spores, Cysts, etc.


  • Thousands of seeds per square meter of soil


  • Dormancy can last for long periods of time
    • protection from harsh environments


  • How does this affect a population?















  • Populations are composites of individual Life Histories
    • who survived from previous year
    • new individuals (births/immigration)
    • survivability of offspring


Survirvorship =

Populations dynamics: Monitoring birth and death



  • Measuring rates of birth and death can tell us if a population is growing or shrinking


  • Cohort Life tables: follow the fate of individuals from a single cohort
    • survivorship
    • difficult for mobile animals


  • Static life table: count # of survivors at different ages

r/K selection theory: Growth and reproduction strategies





  • r-selected species: “cheap” offspring


  • K-selected species: “expensive” offspring



  • These are the 2 ends of a spectrum

Populations dynamics: Movement (dispersal and migration)


Populations with unlimited resources: Exponential growth

Population growth regulation


  • Density dependent
    • competition
    • predation
    • disease
    • resources


  • Density independent
    • weather
    • disturbance


  • Carrying Capacity
    • environmental resistance leads to logistic growth